Biofilters have found industrial applications in a large number of industrial facilities creating noxious odors or volatile organic compounds. Such industries include food processing, paint manufacturing, waste processing, chemical manufacturing, printing, film production, yeast production, poultry processing, hog processing, composting, oilseed processing, solvent users, wood processing, and many others. Biofiltration relies upon a film of microorganisms, commonly referred to as a biofilm, which converts odorous and volatile organic compounds into cell mass, energy, carbon dioxide and water. Biofilters have media upon which the biofilm resides. As airflow moves through the media, the subject compounds or pollutants impinge upon the wet film surfaces surrounding each media particle. This then becomes the feeding or decomposition site for the adsorbed compounds. That is, the microorganisms feed off the adsorbed compounds at these sites. On a micro scale, the mechanism involved in biofiltration is adsorption followed by biological decomposition.
Owing to the heartiness of the microorganisms, biofilters are remarkably robust in their ability to remove pollutants and odors across a wide range of compounds and concentrations. Virtually any biodegradable compound can be removed from an airflow using biofiltration.
Biofiltration differs from carbon adsorption in that the biofilter does not accumulate pollutants and then decline in effectiveness, eventually requiring replacement of the carbon media. The contaminated carbon also must be disposed of in an appropriate fashion. Also, activated carbon is not equally effective for removal of all types of pollutants. Biofiltration is also unlike thermal destruction in that it operates at ambient temperatures, usually requiring no external heat source. Thus, not only does it eliminate a potential source of air pollution, it is also able to function with much less energy input. Biofiltration is unlike wet scrubbers in that no chemicals are required.
Some of the key features of biofilters include low operating cost, no moving parts, robust due to the fact that it is a living system. It is not uncommon to have no chemical requirements, no noise, no dust, no mist, virtually no residual odor, minimal or no corrosion and only periodic maintenance of the active media is required.
Prior art biofiltration systems have drawbacks, however. They typically require a large footprint and they typically provide for low air throughput. Additionally, compaction of the filter media is associated with reduced efficiency due to the channeling of the contaminated airstream around the compacted media, rather than being porous to it. Generally, filter media are of a relatively shallow depth to avoid compaction, about one to four feet or so. Certain prior art biofilters have attempted to overcome these disadvantages, with limited success. Generally, an improvement in one parameter prohibits an improvement in another, or requires additional modifications in order to operate. For example, a relatively high flow rate of 7,500 cfm was achieved in one biofilter, but resulted in a backpressure of 14-15 inches of water. This system utilized only about 21.25 cubic yards of filter media per tank, and the airstream was distributed amongst four separate tanks through a complicated piping system. See Horn, U.S. Pat. No. 5,635,394. In other cases, compaction of filter media is avoided by having multiple media layers sufficiently thin. Layering of media requires corresponding supporting structures and a means to provide for airflow through the supports. See, e.g., Carter, U.S. Pat. No. 5,891,711. In the Carter system, five layers of filter media with a height of one foot each and a total volume of 100 cubic feet (about 3.7 cubic yards) is operated with air flowing at only 200 cfm.
Many conventional prior art biofilters use spray towers to deliver needed moisture to the filter media. Spray towers have a very high water usage, requiring about 50-300 gallons per minute at a 30-100 psi pressure to deliver suitable humidity to the system. Spray towers also contribute to a large footprint. Thus, spray towers, due to their high water usage, have high operating costs, create waste treatment problems and wasted space, which increases capital costs.
Other prior art biofilters provide needed moisture by forcing contaminated air through a water reservoir prior to introduction to the media. See Carter, supra. This system has the disadvantage of additional structures and piping, and also requires manipulation of the airstream for proper humidification, for example, by forcing the air through a fine grate to generate the required diffusion of air into fine bubbles. Variations on this theme include forcing air upward through a packing material, while water flows downward over the packing material in order to humidify the air prior to introduction to the filter media. In some cases, this process occurs in the filter media itself, requiring sprayers as described above.
Accordingly, there remains a need for a biofilter apparatus that is capable of high airflow rates with relatively low backpressure of a large volumes of filter media, while having a small footprint, and with a simple and efficient humidification procedure.
In summary, the present invention consists of a biofilter apparatus having at least one container, a deep, low water flow rate, porous media that minimizes energy requirements to circulate contaminated air through the system and allows for treatment of much greater volumes of air per square foot of filtered media, and an air flow apparatus constructed and arranged to support operation with relatively high airflows and low backpressure. The present invention also consists of a biofilter arrangement that contains a unique air humidification system that allows for humidification of the introduced air using a compact misting system operating under relatively high pressure and low density. The use of the misting further minimizes capital costs by eliminating the need for a spray tower, which is typically used in prior art systems to humidify the airflow. The biofilter arrangements of the present invention allow for biofilters to be stacked six high or more, maximizing utilization of potentially valuable ground space. Capital costs are further reduced by allowing for lower power and therefore less expensive fans to drive air through the system. The present invention also includes methods for utilizing the biofilter arrangements.